370 PHYSIOLOGY OF INDUCED HYPOTHERMIA 



circuit was filled with 150 ml. of donor l)lood. The pump was then started and its 

 speed regulated so as to return 100, 200. or 300 ml. of ])lood per minute. 



Coolinfl in normal dof^s. At a rate of oOO ml./min. through the pump the tem- 

 perature of the returning hlood range<l from 22° C. to 10° C. Shivering began as 

 soon as the pump was started, and continued with increasing severity until con- 

 vulsive seizures occurred, or narcosis from cold supervened. Because of the shiver- 

 ing, the average time required to reduce the rectal temperature of 28° C. averaged 

 two and one-half hours. We could not safely lower the rectal temperature below 

 28° C. because ventricular fibrillation occurred without warning in three of seven 

 dogs (at 26° C. in two and at 27° C. in one). 



When the rectal temperature reached 28° C, circulation through the pump was 

 stopped. To keep the rectal temperature at this level it was sufficient to operate the 

 pump for five minutes every half-hour at a rate of 100 ml./min. Cessation of 

 pumping was followed by a spontaneous rise in temperature of 2° C. per hour, and 

 circulating the blood through the coil immersed in water at 40° C. raised the rectal 

 temperature to 34° C. in about one hour. (Jf four dogs which were rewarmed after 

 hypothermia of six hours duration, two survived indefinitely and two died, one in 

 12 hours and one in 60 hours, both with pulmonary atelectasis. 



The data in table I show that in the unanesthetized dog a rectal temperature of 

 28° C. substantially reduces arterial pressure, respiratory rate, pulse rate and 

 cardiac output and increases the oxygen consumption and the A-V oxygen differ- 

 ence. Although the volume of air breathed per minute per 100 ml. of oxygen con- 

 sumed ("ventilation equivalent") is reduced, the arterial oxygen content remains 

 normal. 



Cooling in hypovolemic dogs. Dogs were bled into an elevated reservoir from 

 the femoral artery until the arterial pressure fell to 30 mm. Hg. This level was 

 reached about five minutes after bleeding was started. Cooling was begun at this 

 time and a rectal temperature of 28° C. was reached within an hour. Starting the 

 pump induced an abrupt return of about 100 ml. of blood from the reservoir to the 

 dog. On stopping the pump this blood returned to the reservoir. Shivering did not 

 occur. The dogs were quiet and weakly responsive to ordinary stimuli. 



Of a total of six experiments, four were not completed because of technical 

 difificulties in two and ventricular fibrillation in two others (in one at 34° C. and in 

 another at 32° C). In the two completed experiments the hypotensive period was 



TABLE I 



Cardiovascular and Respiratory Dynamics in Seven Unanesthetized Dogs Prior 

 TO and During Hypothermia (28° C.) by the Veno-venous Cooling Method of Ross" 



The {lata are averages. 



